1. Field of the Invention
The present invention relates generally to epoxy resins and the properties of such resin when they are burned. More particularly, the present invention involves improving the burn properties of high performance epoxy resins by reducing the amount of sulfur-containing compounds that are emitted when such resins are burned and by also reducing the time it takes the epoxy resin to self-extinguish once it begins to burn.
2. Description of Related Art
Epoxy resins that are reinforced with a fibrous material, such as glass or carbon fiber, are used in a wide variety of situations where high structural strength and low-weight are required. Composite materials that use a high performance epoxy resin matrix are especially popular in the aerospace industry where weight and structural strength are important engineering and design considerations. High performance epoxy resins typically include one or more thermoplastic materials that provide “toughening” of the epoxy resin. Although such high performance epoxy resin composite materials are desirable because of their relatively high strength to weight ratio, they do present some specific issues with respect to flammability, toxic emissions and other burn properties.
High performance epoxy resins of the type that are used in the aerospace industry are generally cured at temperatures of about 177° C. These high performance epoxy resins typically include thermoplastic tougheners and curing agents that contain sulfur. These types of epoxy resins tend to generate toxic sulfur-containing gases when they are burned. The generation of toxic gas is of particular concern when epoxy resin composite parts are located in the interior of aircraft or other aerospace vehicles. A major goal in developing formulations for such high performance epoxy resins is to limit the amount of sulfur emissions that are produced upon burning, while at the same time not reducing the structural strength of the cured composite part. It is also important that any attempt to reduce sulfur emissions does not adversely affect properties of the uncured epoxy resin, such as tack and viscosity. The tack and viscosity of the uncured resin are especially important when the epoxy resin is used to make prepreg, which is a common intermediate material used in the fabrication of aerospace parts.
Resistance to surface flammability is also an important area of concern for high performance epoxy resins. It is important that an epoxy resin composite part, which is on fire, be able to self-extinguish once the source of heat and/or flame is removed. It is also a significant goal of epoxy resin formulators to develop epoxy resins that are self-extinguishing in as short a time period as possible, while at the same time keeping structural strength of the finished composite part at the levels needed for aerospace applications. The same requirement that the tack and viscosity of the uncured epoxy resin not be adversely affected applies with respect to attempts to formulate epoxy resins with low self-extinguishing times.
Sulfur, which is principally in the form of sulfur dioxide (SO2), is a toxic compound that is emitted when a high performance epoxy resin part burns. The National Bureau of Standards (NBS) Smoke Density Chamber is a standard combustion test system that is used to measure SO2 emissions, as well as emission levels of carbon monoxide (CO), hydrogen cyanide (HCN) and nitrogen oxides (NOx). The SO2 emission level is determined by measuring the amount of SO2 in parts per million (ppm) that is released by a sample during combustion under specified thermal exposure conditions in the NBS Smoke Density Chamber.
The specific combustion parameters that are used for measuring the SO2 emission levels of high performance epoxy resin composite materials are set forth in BOEING Specification Support Standard BSS-7238 (Revision B) and BSS-7239 (Revision A), which is recognized in the aerospace industry as a standard test method. The SO2 emission levels for a typical high performance epoxy resin that contains polyether sulfone (PES) or other sulfur-containing thermoplastic toughener will be over 100 ppm. An exemplary PES-toughened high performance epoxy resin is HexPly® resin 8552, which is available from Hexcel Corporation (Dublin, Calif.). It would be desirable to provide epoxy resin formulations where the SO2 levels are 100 ppm or lower.
The United States Federal Aviation Administration has established regulations and requirements for fire resistance of aircraft interior parts and materials. These requirements are set forth in FAR Part 25, Appendix F, Part 1. One requirement is that the material be able to self-extinguish once the flame source is removed. The test procedure for measuring the self-extinguishing time for epoxy resins are set forth in the FAA Aircraft Materials Fire Test Handbook and in BOEING Specification Support Standard BSS-7230 (Revision H), which is recognized in the aerospace industry as a standard test method. A modified version of Method F of BSS-7230 (Revision H) involves igniting a vertically oriented sample of cured neat resin by exposing it to an ignition source for 10 seconds and then measuring the time it takes for the sample to self extinguish. It would be desirable to provide high performance epoxy resin compositions where the self-extinguishing times are as short as possible.